Low-Cost Cabinet Solution for Prototype Systems

Would cabinets made using a custom extrusion technique be good enough (and worth the cost) for creators of prototype electronic products?

Both the creators of hobby projects and the creators of professional prototypes share a common problem -- what to do about the cabinet. In the not-so-distant past, creating a prototype printed circuit board (PCB) was the most time-consuming and expensive problem, but now this is much less of an issue. By comparison, acquiring an affordable, professional-looking cabinet remains an expensive undertaking.

Recently, I've been chatting to someone who thinks he has a solution -- EETimes community member Charles Spence. In a recent telephone conversation, Charles was saying that he is thinking of launching a Kickstarter project, but first he would like to "hoist his idea up the flagpole and see how well it flies," as it were, and that's where you come into the picture.

I personally really like Charles's idea, so I asked him to write it up in more detail so I could share it with you. Charles responded as follows:

Dear Max, thank you for giving me the opportunity to share my idea with other members of the EETimes Community. One part of building prototype electronics that continues to plague me is the cabinet. It still seems to be the remaining piece for the hobbyist and professional that forces their work to look amateurish to family and friends or to the customer. While I admire the quick and dirty methods Jim Williams and Bob Pease taught us, only the enlightened few seemed to deeply appreciate the ingenuity of a cookie tin for EMI shielding. However, it takes an understanding wife or boss to accommodate the hoarding of these likely Christmas gifts.

While I am also a fan of the "dead bug" method of circuit prototyping, it tends to hide significant issues with regard to parasitic capacitance or leakage paths that -- if you ever want to replicate your circuit -- will bite you. I also argue that creating a decent cabinet is more time-consuming than creating a prototype PCB. This is because you can go straight to a PCB for little relative cost these days (when I started my first real electronics job, it was $1K for a set of two-layer proto-PCBs; now it's only around $120 on the high end). I create a PCB for all my prototypes -- what seems to remain is the box.

On my last project, I think I came up with a solution I would like to share with your readers. This solution applies principles similar to those that enable low-cost prototype PCBs and even low-cost integrated circuits -- share the panel or wafer.

Current industrial CNC laser cutters can slice up a panel of aluminum quickly and cheaply. What seems to plague the sheet metal business is making a bend cheaply enough for use in prototype cabinets. A simple aluminum extrusion gets past this issue, which inspired the following cabinet design.

This example represents a 3" x 5" x 7" deep cabinet. The panels are intended to be cut from 0.050" thick 5052-H32 sheet aluminum. For the purposes of this example, the extrusions are shown in orange in an effort to make their design more visible. These extrusions are intended to be made from 6061 aluminum hardened to T5 or T6. The extrusions do include a guide channel for a PCB, but the PCB(s) could also be mounted to any panel via standoffs and screws.

In one implementation, the bottom and two side panels could be glued to the extrusions with a cyanoacrylate (Krazy Glue I) like Loctite 380 (Black Max) or an adhesive transfer tape like 3M 465 or 966. The top panel would still be allowed to slide to facilitate easier access to the internal electronics for debugging and testing.

The cheapest way to make this type of cabinet uses raw aluminum. My current estimates show they could be made with custom cutouts for $50 each in quantities of one, assuming several people ordered cabinets at the same time (each cabinet can be different). Furthermore, since the un-assembled cabinet can be shipped flat, it can be sent via USPS Priority Mail bubble pack to most places in the US for less than $6.00.

Observe that this basic design can be extended all the way up to 19" rack-mount cabinets. The front panel can be extended to have rack mounting holes (but it would also need to be made from thicker aluminum, which would drive the price up a bit). A simple bracket could be mounted in the middle to stiffen the center section for cabinets this wide.

@Charles: I'm thinking that if you did this via a Kickstarter project, you could raise the money to create an intuitive and easy-to-use design feature on your website.

Here's the way it could go. First, the user enters the outer dimensions of the cabinet they want to create (the used can specify units of inches or mm/cm).

Taking the dimensions of the extrusions into account, the system uses the user-provided dimensions to automatically create the outlines/templates for the top, bottom, side, and end panels.

In the case of expert users with access to 3D mechanical CAD packages, these users could take the generated panels and modify them to their heart's content.In the case of novice/hobby users, you could provide a drag-and-drop feature to create holes in the panels -- you could have circles, ellipses, rectangles, rectangles with rounded corners, etc. After dragging one onto a panel, the user could specify dimensions (e.g., D diameter, W wide, T tall, etc.; also X-Y offsets from the lower-left corner of the panel. You could also provide cut/copy/paste type facilities -- also shapes for common connectors and suchlike. Maybe do something similar for the silkscreen/annotations... Just a thought...

I'm pretty sure that a very similar design is commercially available (at least all of the structural elements, not including customization). I have several "test benches" (actually simulators) built using COTS extrusions etc. The supplier is almost certainly German so the stuff might be quite a bit pricier than your projections. I'll try to get a photo or two that show the structure without revealing any proprietary info of the test benches proper.

I am not sure if what you are offering is the low cost machining of the housing or the housing itself.

If it is the housing then I am afraid I am going to rain on your parade. There is at least one manufacturer of simlar housings (and they offer machining as well) Rose Bopla. Here is their offerening in extrusions. It seems that the BOS Ecoline is similar to what you are proposing, if I understand you correctly. One range had endplates that allowed for an octal relay base.

Some years ago we produced a "bulletproof" switchmode power supply in a one piece extrusion which made for easy IP rating (except for the end plates). Have you considered whether this is a market you want to support.

We also had a customer who designer an "L" shaped extrusion which when one side was flipped gave you the rectangular cross section. It was held together with the endplates which were in fact heavy duty connectors like these shells and a sutiable multipin insert.

In all of these however, I have found debugging when the unit is inside any extruded housing to be difficult to say the least.

I am trying to solve custom maching of the housing, total cost (except NRE) and turn around time. I am trying to address prototype designs where you just need one maybe three.

Everytime I have looked at the extrusions you mentioned I find minimum and stocking issues. In addition, the ones that you provided a link to appear to still need a front panel machined at a minimum. If you need any additional holes cut into the side panels for a one-off I imagine the cost are significant. If you do not need holes in the cabinet for connectors etc. then off the shelf cabinets seem like the obvious low cost option. I always seem to need holes in my cabinets.

The housing is not extruded just the corners. It alows making cabinets as small as 1.5"x1.5"x1.5" to rack mount monsters using the same corner extrusions. You just cut the six side panels to the dimensions needed. The extruded housings you mentioned are usually restricted in one or two dimensions.

This design also solves the debugging problem. You can remove as many sides as you need if you mounted your PCB to the base using standoffs. I feel it is a very flexible system.

I looked everywhere for extrusions like the ones shown in my sketch. Could not find them. The closest are used for making glass display cases. They only work with panels 1/8" or thicker. I have looked through Maytec's catalogues for other projects. I associate them with alternatives to 80/20. I did not know they had an extrusion that could accomodate a 0.050" think panel without some kind of spacer. If they have something similar to the one I propose could you provide a part number or link? The extrusion that I propose costs from $0.80 to $4.00 per foot depending on how much you order from a US extruder. Anodizing is extra of course.

Really like the GUI idea. Coding up a panel generator based on overall cabinet seems straightforward. However, dxf file format scares me. The examples I have looked at show lots of dendrite. Do not know what is important for reliability/compatibility. Cadenas Part Solutions? DesignSpark?

I'm not an expert on these parts; I just Googled Maytec today based on the name on one of the plastic corner plugs on my bench and looked at a couple of part numbers. There is a "part chooser" app on their site, and also a US$ price lst. I was surprised at the relatively low cost for these German parts. The small cross-section extrusions comparable to yours (and used on my benches) run between US$5 to 10 per METER in unit quantities. I have seen similar stuff from several suppliers in the past, and in fact have a mid-size portable equipment chassis in my basement that bears a very strong resemblance to your sketch. I bought it surplus many years ago for a few $.

I tried to post an image that compares the extrusions that you mentioned plus another with what I am proposing. I cannot seem to be able to place it in this reply but I have placed it on postimg.org for others to see.

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I can see making the MayTec parts work, but they seem a compromise. One costs significantly more because it requires a machining operation. The other still seems more expensive than what I propose (~$3.90/ft). They are made and stocked in the US.